[FIX] website, *: fix conversion of specific svgs on Firefox

*: website_slides

A function allowing to convert a SVG to a PNG has been added as an
utility function in website.utils and is used for the upload of document
in website_slides. This function add support for a specific case which
was not supported by the previous conversion function used for the
document upload in website_slides: SVG without width and height
attributes on Firefox.

Note: this should be simplified in master, the fix here is done the most
stable way possible.

Related to task-2602521

closes odoo/odoo#75741

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: qsm-odoo <qsm@odoo.com>